This invention relates to a biaxially oriented polyester film and a production method thereof. Particularly it relates to a biaxially oriented polyester film suitable as a base film for high density magnetic recording media and a production method thereof.
Biaxially oriented polyester films are used for various applications in view of their excellent thermal properties, dimensional stability and mechanical properties, and their usefulness as base films, for example, for magnetic tapes is well known. In recent years, base films for magnetic tapes are required to be further thinner and to allow higher density recording, for allowing apparatuses to be reduced in weight and size and for allowing recording for longer periods of time. However, a thinner film is insufficient in mechanical strength and less firm as a film or is likely to be elongated. Therefore, for example, if it is used for a magnetic tape, the recording track is likely to deviate, and the head touch may be worsened to lower the electromagnetic conversion properties disadvantageously. Therefore, there are more intensive demands for improving such properties as the tape elongation deformation caused by tension, the dimensional stability in tape use environment, traveling durability and preservability.
As base films to respond to the above demands, aramid films have been used in view of strength and dimensional stability. Aramid films are expensive and disadvantageous in view of cost, but they are used since there is not substitute for them.
On the other hand, higher strength polyester films (e.g., films of JP, 42-9270, B, JP, 43-3040, B, JP, 46-1119, B, JP, 46-1120, B, JP, 50-133276, A, U.S. Pat. No. 4,226,826, etc.) obtained with the prior art have many problems to be solved such that (1) the tape is cut during use, (2) edge damage occurs due to insufficient rigidity in the transverse direction, (3) the recording track deviates to cause errors during recording and reproduction and (4) because of insufficient strength, a thin film cannot be formed and desired electromagnetic conversion properties cannot be obtained. For application as a tape for large capacity high density recording, especially the problem of (3) is most important.
This invention has been achieved as a result of examining to solve the above problems of the prior art.
The object of this invention is to provide a biaxially oriented polyester film suitable as a base film for high density recording media, small in track deviation and excellent in traveling durability and preservability, and also to provide a production method thereof.
The inventors studied intensively to solve the above problems, and as a result, found that if the dimensional change rate of a biaxially oriented polyester film in the transverse direction under the tension, temperature and humidity conditions assuming a tape use environment is kept in a specific range, the magnetic tape obtained by using the biaxially oriented polyester film can be decreased in recording tack deviation and improved in traveling durability and preservability. Thus, this invention has been completed.
The object of this invention can be achieved by the following constitution.
(1) A biaxially oriented polyester film, characterized in that the dimensional change rate (A) in the transverse direction of the film is in a range of xe2x88x920.3 to 0% when the film is allowed to stand at 49xc2x0 C. and 90% RH for 72 hours, while being loaded with 32 MPa in the machine direction.
(2) A biaxially oriented polyester film, according to (1), wherein the ratio of the dimensional change rate (A) in the transverse direction under the conditions of (1) to the dimensional change rate (B) in the machine direction respectively in absolute value (|A|/|B|) is in a range of 0.1 to 1.0.
(3) A biaxially oriented polyester film, according to (1), wherein the heat shrinkage in the transverse direction at 100xc2x0 C. is in a range of 0 to 0.5%.
(4) A biaxially oriented polyester film, according to (1), wherein the sum of the elastic modulus in the machine direction and that in the transverse direction is in a range of 9 to 30 GPa.
(5) A biaxially oriented polyester film, according to (1), wherein the thickness variation in the machine direction is 5% or less.
(6) A biaxially oriented polyester film, according to (1), which contains a polyether imide.
(7) A biaxially oriented polyester film, according to (6), wherein the content of the polyether imide is 5 to 30 wt %.
(8) A biaxially oriented polyester film, according to (7), which has a single glass transition temperature.
(9) A biaxially oriented polyester film, according to (1), wherein the polyester is polyethylene terephthalate, polyethylene-2,6-napthalene dicarboxylate or a copolymer thereof or a modification product thereof.
(10) A biaxially oriented polyester film, according to (1), wherein the halfwidth in the circumferential direction of the diffraction peak of the crystal face in the principle direction of the polyester, obtained when the polyester film is revolved around its normal, in the crystal orientation analysis by wide angle X-ray diffractometry, is in a range of 55 to 85 degrees.
(11) A method for producing a biaxially oriented polyester film, in which a cast film is stretched in the machine direction and in the transverse direction, stretched again in the machine direction and/or in the transverse direction, and heatset-treated and relaxation-treated, characterized by executing the relaxation treatment in two or more stages at a total relaxation rate of 5 to 10%.
(12) A method for producing a biaxially oriented polyester film, comprising the steps of stretching a cast film in the machine direction and in the transverse direction, stretching at a small ratio of 1.01 to 1.3 times in the transverse direction at a temperature in a range of glass transition temperature (Tg) to Tg+50xc2x0 C., and stretching again in the machine direction and/or in the transverse direction.
(13) A method for producing a biaxially oriented polyester film, in which a cast film is stretched in the machine direction and in the transverse direction, stretched at a small ratio of 1.01 to 1.3 times in the transverse direction at a temperature in a range of glass transition temperature (Tg) to Tg+50xc2x0 C., stretched again in the machine direction and/or in the transverse direction, and heatset-treated and relaxation-treated, characterized by executing the relaxation treatment in two or more stages at a total relaxation rate of 5 to 10%.
(14) A method for producing a biaxially oriented polyester film, according to any one of (11) through (13), wherein when the cast film is stretched in the machine direction and in the transverse direction, it is stretched in two or more stages in the machine direction and/or in the transverse direction.
(15) A magnetic recording medium, using the biaxially oriented polyester film as set forth in any one of (1) through (10).